The proton motive force determines Escherichia coli's robustness to extracellular pH

Abstract

Maintaining intracellular homeostases is a hallmark of life, and key physiological variables, such as cytoplasmic pH, osmotic pressure, and proton motive force (PMF), are typically interdependent. Using a mathematical model, we argue that near neutral pH homeostasis implies that cells must export ions other than protons to generate physiological electrical potential across their plasma membrane. For Escherichia coli, proton:ion antiporters are the only known cation efflux pumps, and we therefore predict that principal function of antiporters is to generate an out-of-equilibrium plasma membrane potential and so maintain the PMF at the constant levels observed. Consequently, the strength of the PMF determines the range of extracellular pH over which the cell is able to preserve its near neutral cytoplasmic pH, and the non-zero PMF is needed to maintain membrane potential. In support, we concurrently measure the PMF and cytoplasmic pH in single cells and demonstrate both that decreasing the PMFs strength impairs E. colis ability to maintain its pH and that artificially collapsing the PMF destroys the out-of-equilibrium plasma membrane potential. We further predict the observed ranges of extracellular pH for which three of E. colis antiporters are expressed, through defining their cost by the rate at which they divert protons from being imported to generate ATP. Taken together, our results suggest a new perspective on bacterial electrophysiology, where cells regulate the plasma membrane potential to maintain